Special Topics in Schematic CaptureAugust 23, 2019 by Stephen Colley
Learn about heterogeneous and homogenous buses, special IC packages, and more!
Learn about heterogeneous and homogenous buses, special IC packages, and more!
This article is a collection of special topics about the creation of schematic diagrams. The topics are ones I have carried around in my head for years. I hope you find them useful.
This article assumes experience with building schematic symbols and creating basic schematics.
High Pin Count Parts
Here is the highest pin count symbol I found from a quick search of KiCAD libraries. To me, this is too dense and totally impractical. There should be a rule against this sort of thing! I went back over a bunch of my schematics and found my threshold is around 100 pins. Over 100 pins and I always created multiple units per part.
I break up a high pin count part by splitting the pins into functional groups. For example, an FPGA will have one unit for all the power and ground pins. Another unit will have all the pins associated with configuring the part. Then, there are multiple units for the I/O pins.
Here is a unit for the configuration pins of an FPGA.
This symbol can be placed anywhere in the schematic that makes sense. In this case, it is with the configuration memory IC and the JTAG connector used for programming and debugging. Also on the sheet are several miscellaneous things which needed a home.
Here is the unit with the power and ground pins.
There is not much structure to the power pins. Two types of power pins are in two groups: internal power in one group and I/O power in another group. Other devices have much more complex power structures and would need more groups. This unit is on a sheet with voltage regulators and power-monitoring circuits.
The I/O pins are split into four units. Here is one unit showing about 25% of the I/Os.
Altera (now Intel) parts often have internal interconnects with a prominent row and column structure. In this part, the I/O speed is faster for I/Os connected to row interconnects and slower for I/Os connected to the columns. Since this was a high-speed design, I needed to consider which signals were connected to rows or columns, and I created a symbol that helped me in the pin assignment. You can see that this unit is all “row” pins. In addition, the rows are further grouped together (ROW A, B, C, etc.). This arrangement indicates other timing characteristics inside the device.
Consider grouping I/Os in programmable devices to reflect the functionality of the part in a particular design. The extra work for custom symbols is OK with me if it helps in the design and understanding of the circuit.
In general, schematics larger than one sheet can be organized in two ways, flat or using hierarchy. Flat schematics use connections directly from sheet to sheet. A signal typically goes off the side of one sheet and appears on the side of another sheet. Hierarchy uses a “hierarchical block” in a sheet to represent another sheet. Connections to the sheet are done by connecting to the block. There can be multiple hierarchical blocks on a sheet. The organization of blocks and sheets can look a lot like a file directory in a computer. There is usually a “root sheet” which is a lot like a root directory.
Hierarchy is used for two main purposes. First, it provides for a logical structure within the schematic and a way to navigate between sheets in a way that is consistent with signal connections. Second, it allows circuitry to be drawn once on a sheet but repeated multiple times by using multiple hierarchical blocks that reference the same sheet.
Using hierarchy is a powerful way to organize a design. It takes a bit of work to get comfortable with this sort of structure. However, for me, it has become essential. I use it so much that I never have the direct sheet-to-sheet connections used in a flat design. A sheet is always connected through a hierarchical block. The subtle effect is the schematic takes on the appearance of a three-dimensional block diagram. I like the way this documents the design and makes it quick and easy to find things.
Here is the root sheet of a 20-channel high-voltage amplifier board. Of course, it is not possible to see detail. However, perhaps you can see how the hierarchical blocks look like a block diagram.
I provide notes to list what circuitry is in each block. Clicking into a block makes it easy to find something. Not here but in other designs, I included blocks on the root sheet with no circuit connections but referencing sheets with notes and diagrams. Using blocks like this works like a table of contents with links.
I notice an I2C bus in the middle with three signals: I2CSDA, I2CSCL, and I2CINT. I wish I had been able to use a “heterogeneous bus” which is the next topic! I could have condensed three lines into one!
Homogeneous and Heterogeneous Buses
All the schematic programs I know support collecting multiple, similarly named nets into a “bus”. The term originally came from bundling signals in a data bus or address bus. In the schematic below, this feature bundles signals with the name “SHDN” followed by sequential numbers. This is called a “homogeneous” bus.
In the middle of the schematic is an SPI interface with four signals: SS, SCK, SDI, and SDO. They are a functional group, and it would be handy to combine them and use one line and one name for all of them. Some schematic programs allow signals to be grouped into a “heterogeneous” bus. There doesn’t seem to be a standardized name for this type of bus. Altium calls it a “Signal Harness”. OrCAD uses the term “NetGroup”. In Eagle all buses are heterogeneous. KiCad is developing this feature and calling it a “Group Bus”.
Here is a heterogeneous bus created in Eagle for the four nets in the schematic.
In my opinion, a heterogeneous bus provides logical structure to the schematic and reduces clutter.
Special IC Packages
Mechanical designers have become quite clever in solving packaging issues related to smaller and smaller form factors and thermal management. However, some of their designs cause connections that should be shown on a schematic. I try to include in the schematic any package element that connects to a net. For example, a plated-through mounting hole for a connector shell or a thermal tab on a part that connects to GND or power should be in the schematic. Here are two examples of packages with features connecting to power or GND.
The part on the left has a metallic tab that is usually soldered to a copper area on the top of the board to dissipate heat. The part on the right has an “exposed pad” on the bottom that is soldered to a copper area. In both cases, the tab or pad is often electrically connected inside the IC.
This schematic shows how I handled the exposed pad for a voltage regulator using the package on the right.
Pins 1 to 5 are the normal pins. I added Pin 6 to represent the electrical connection to the exposed pad and connected it to GND. I also included 6 thermal vias that conduct heat from the top to the bottom copper areas. The vias are included in the schematic because they make an electrical connection. Here is the result:
Question: When is a 64-pin part not a 64-pin part?
Answer: When it is an “E64” package.
When building a symbol for this part you immediately see there are no ground pins on the part! One commenter in an Intel forum speculated it was a cutting-edge part using a quantum effect and no ground connection was needed. Well, not quite. Ground is an exposed pad on the bottom of the part. Here is the drawing of the bottom with the pad in the middle.
To deal with a part like this, just add a Pin 65 to the symbol. Then, connect it to GND in the schematic. Be sure to add a note on the schematic! The datasheet points out that the pad is only for electrical connection and has no thermal function. The PCB designer will need to know that!
“Back in the old days” schematic diagrams were hand drawn, most often by a professional draftsman. From time to time, they might take a break and doodle on the schematic about something that sparked their imagination. Here is an example from a Tektronix schematic.
Image used courtesy of vintageTEK
Have you ever added a cartoon or emoji to a schematic?
Have you ever used a non-orthogonal line for a net? Does your schematic program allow them?